portable: allow reattaching when one image has a version and the other does not

portable: allow reattaching when one image has a version and the other does not

A reattach might go from img.raw to img_0.1.raw or viceversa, but this is
not allowed right now as we try to match the full name.

Also take into account that running strcspn(a, '/') on an image name, without
leading path, will return the length of the full string, but the versions
might be different so they won't match, eg:

img_0.1.raw -> 12
img_0.1.1.raw -> 14

So adjust the check to take that into account, and skip it if we are not
dealing with directories